Many electric motors used in industrial and commercial processes and buildings are controlled by softstarter equipment.
A softstarter device is in this case a motor controller adapted for controlling a motor during the start-up phase and stopping phase of the operation of the motor. The softstarter device is used to start and stop the motor in a desired manner, such as to eliminate electrical surges in the electrical supply and/or overheating in the motor. Softstarters are also used to reduce or eliminate mechanical shocks or vibration which might otherwise occur under start/stop conditions causing wear and damage to the pumps, fans etc. driven by the motors. A softstarter device typically measures the input current and/or voltage of the motor and regulates the input current and/or voltage received by the motor to achieve a desired start or stop performance. The soft-starter usually comprises semi-conductor switches or the like and logic units to control the switches. A softstarter device is sometimes also called a transient control starter.
The softstarter device generally requires configuration to suit the circumstances of each application. The parameter configuration of the softstarter device may be grouped as motor-data, describing the present motor and the start-stop parameters describing the start and stop process. During configuration the parameters influencing in particular the starting current, and also factors such as the speed, start up time or maximum input current etc. have to be selected or set up in the softstarter device according to what kind of start or stop is desired for the motor and/or the equipment it is driving. Softstarters may comprise many features and functions and the configuration is often therefore somewhat difficult and time consuming.
A softstarter device normally uses a reduced-voltage stopping technique ensuring a continuous voltage decrease supplied to the motor during the stopping phase of the motor instead of switching off the voltage supply directly. This is used to control the performance of the motor during the stopping phase. As the torque is reduced, the speed of the load will reduce to the point where the load torque equals the shaft torque.
During start and stop of motors used as pumps for pumping water in pipe-systems problems due to mechanical stress are common. These problems include drive belt slippage, wear on gear boxes and so on. A well known problem due to mechanical stress is the occurrence of high-pressure water shocks in the pipes. The phenomenon is also called water hammering. Water hammering occurs when the water flow is shut off suddenly which sends a pressure or shock wave down the water line through the water shocking the pipe system and creating the hammering noise. The involved forces may be high and the stress on the pipe-system may therefore be great causing damages to the pipe-system. The consequence is higher maintenance costs and also a shortening of the lifetime on the pipe-system.
To solve this problem conventional softstarters use voltage control to limit the motor input current and torque. Typically, the voltage supplied to the motor in this solution is a linear voltage ramp. The voltage is continuously decreased during the stopping process. However due to the dynamic characteristics of a water system and the non-linear characteristics of the motor this does not put a stop to the water hammering in the pump system.
Therefore many softstarters of today comprise a voltage step down to limit the voltage in the beginning of the stop ramp. The drawbacks of this solution are that it is difficult to set up the configuration parameters for all situations. To use a static setting to control a dynamic system is not efficient, because, when the system changes such as the water level or the pressure of the water, or otherwise operating the pumps the configuration settings are no longer accurate and will not prevent the water hammering.
Examples of softstarters for controlling acceleration (start-up) and/or deceleration (stopping or run down) illustrating the above can be found in U.S. Pat. Nos. 5,859,514, 6,667,596 and 4,707,650. U.S. Pat. No. 5,859,514 is related to a start-up process and U.S. Pat. No. 6,667,596 is related to a run down process.